There are 2 words which mean the word “because” in German:

• weil
• denn

They affect the sentence structure differently.

“Denn” uses a main clause, and doesn’t change the “default” sentence structure.
“Weil” uses a subordinate clause, so the verb is moved to the end of the sentence/clause.

e.g.
Er ist müde, denn er kann nicht schlafen. ➡️ He is tired because he cannot sleep.
Er ist müde, weil er nicht schlafen kann.         ^

Also, you cannot start a sentence with main clause with denn.

e.g.
✅ Weil er nicht schlafen kann, ist er müde. ➡️ Because he cannot sleep, he is tired
❌ Denn er kann nicht schlafen, ist er müde.     ^

Weil is more commonly use in spoken German.

1. German StackExchange: Difference between “weil” and “denn”